The present invention pertains to a multispectral color image reproduction system.
Multispectral color image reproduction systems have been known for a rather long time and they represent an alternative to the prior-art processes of the color image reproduction technique with three color channels called three primary systems below for the scanning, transmission and reproduction of color value signals, e.g., on a display screen. Color values are understood as color values of the CUE 1931 standard observer such as XYZ or linear transformations of them such as RGB throughout the text below:
The primary object of the present invention is to substantially reduce the large amount of data for representing every individual spectral color stimulus of the color captured by an observer without the noticeable loss of the color information visible to the observer.
The primary object according to the present invention is achieved by the linear multispectral values representing spectral color stimuli being nonlinearly distorted in a coding system adapted to the human color distinction capacity, wherein the distortion is made dependent on all or individual dominant amplitudes of the linear multispectral values and the distortion is inverted in a decoding system.
The present invention can be embodied by means of multidimensional look-up tables or by means of a system with distortion and inverted distortion functions controlled in a stepwise manner. An embodiment with predictive coding, in which only the differences of nonlinearly distorted amplitudes are transmitted to estimated values, is particularly advantageous.
According to the invention, a multispectral color reproduction system is provided with a recording unit for measuring the spectral colors stimuli with a spectral color scanner and a coding and decoding system for an efficient digitized display of the spectral color stimuli by multispectral values in a data format of K values, wherein the first three values can be linear combinations of color values determined by weighing the color stimuli with the color matching functions of the CIE 1931 standard observer to preserve compatibility with conventional three primary color reproduction technique and the linear multispectral values are weighting values of a linear decomposition of defined basis functions and the basis functions may also be defined orthogonally. The K linear multispectral values are additionally distorted nonlinearly in the coder and are then transmitted at shorter binary word length and are undergoing a nonlinear transformation again in the decoder, the distortion characteristic of the decoder being the inverse character of the decoder, and the distortion characteristic of every individual value can be made dependent on all other multispectral values.
The nonlinear distortion and the assignment to binary coded words in the coder and the inverted distortion in the decoder may be built up by means of a K-dimensional table with the linear multispectral values as input addresses and with the distorted values as entries to the addresses and the inverted distortion is performed by means of a corresponding inverse table.
The nonlinear distortion and the assignment of binary words and the inverted distortion into K linear multispectral values in the decoder may be performed stepwise, starting with the multispectral value having the greatest effect on perceptibility, and all other multispectral values can be quantized according to declining effect on perceptibility and as a function of the previously distorted values and the corresponding inverse operations are performed stepwise in the decoder.
Three of the linearly coded input values of the nonlinear distortion may be a linear transformation of color values determined from the color stimuli, e.g., the standard color values XYZ or the RGB values of a standardized RGB color space, and thus there is compatibility with conventional primary systems due to three multispectral values.
A stepwise coding and decoding may be performed corresponding such that estimated values are formed from the already coded values for the respective values of higher order, and only the differences between the estimated values and the current values are transmitted and the estimated values are again added stepwise to the differences in the decoder and the output values are formed from this as multispectral values after inverted distortion.
Contrary to prior-art color scanners with three color scanning channels and three color signals at the output, the full spectral information of every picture element is consequently represented here by scan values, and the number of scan values may be on the order of magnitude of N=9 to N=16 or even N=32 in practice. A detailed description of a multispectral scanner can be found, e.g., in the patent DE 41 19 489 A1 which is hereby incorporated by reference.
The advantages of this recording technique over the prior-art technique with three color channels are also described in this patent in detail. These are essentially the avoidance of a number of systematic problems which arise in the three-channel technique due to an insufficient imitation of the theoretical color mixing curves and due to the mesmerism properties of the colors in images or natural scenes. The above-described multispectral technique can also be used, in principle, for the reproduction of natural images instead of original images.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.